1. Field of the Invention
The present invention relates generally to an antenna array device and a method thereof in a mobile communication system, and in particular, to a device and method for forming a transmission beam.
2. Description of the Related Art
As the number of mobile subscribers drastically increases, the capacity of the mobile communication systems approaches a saturation point. This means that mobile communication systems are in need of more advanced applications to increase the system capacity, particularly the capacity of a forward link for diverse multimedia services.
The capacity of the forward link can be increased by designing an efficient transmission antenna array system If the mobile systems use only single transmit antennas, for example dipole antennas, transmission signals are propagated in all directions. In this situation, when a transmission is performed toward a desired specific mobile station through a dedicated channel, as opposed to a situation where transmission to all mobile stations is performed using a base station transmission antenna through a common channel, much of the radiation energy except radiation energy for the specified mobile station is useless and the unnecessary radiation energy becomes interference signals to other mobile stations. If the base station transmits a signal only in the direction of the specific mobile station for communication on the dedicated channel, good communication quality is ensured with low transmission power and interference to other mobile stations is decreased. Consequently, the capacity of the forward link increases.
This effect can be achieved using a plurality of antennas. A transmission/reception device related with the antennas is called a transmission antenna array system or transmission smart antenna system. While the transmit antenna array system is applicable to various mobile communication fields, it will be described here in context with CDMA cellular mobile communication for convenience sake.
The structure and operation of a transmit antenna array in the mobile communication system will be described hereinbelow.
FIG. 1 illustrates the transmission beam formation in the transmit antenna array. Referring to FIG. 1, let a transmission signal from a base station be s(t). The signal s(t) is duplicated into a plurality of identical signals, the duplication signals are multiplied by corresponding complex weights in multipliers 11l to 11L, and the resulting signals are transmitted in the air through the respective antennas. A mobile station, using a single antenna, receives the sum of the transmission signals that the base station transmits through the antennas. A direction in which each transmission signal is directed is determined by a weight multiplied by the transmission signal and the geometrical structure of the transmit antenna array. The reason for assuming that a single antenna is used in the mobile station is that the mobile station does not typically use an antenna array due to limitations of cost, size, and mobility.
Suppose a linear antenna array has L antennas as shown in FIG. 1 and each antenna has a complex weight ωi(i=1, 2, . . . , L), a signal transmitted in a direction θ is proportional towHa(θ)  (1)where w=[w1w2. . . wL]T is a weight vector,
            a      _        ⁢          (      θ      )        =            [              1        ⁢                  ⅇ                      j2            ⁢                                                  ⁢            π            ⁢                                          d                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                θ                            λ                                      ⁢        K        ⁢                                  ⁢                  ⅇ                      j2π            ⁢                                                            (                                      L                    -                    1                                    )                                ⁢                d                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                θ                            λ                                          ]        T  is an array vector, H represents Hermitian, T represents transpose, d is the distance between antennas, and λ is the wavelength of a carrier frequency. The array vector refers to the relative strength and phase of a signal transmitted from each antenna to a remote destination in the direction θ, as expressed in vectors.
wHa(θ) is greatest when w and a(θ) are in the same direction and wHa(θ) is 0 when w is at a right angle with a(θ). Therefore, the strength of a transmission signal varies according to the transmission direction θ. On the same principle, a signal can be transmitted with the greatest strength in a specific direction θ by controlling w.
An antenna array is different from a diversity antenna device in that it transmits a signal in a particular direction. The distance between antennas (wavelength order length) is shorter in the antenna array than in the diversity antenna device.
In general, an antenna array is provided to a base station that can accommodate a plurality of antennas and controls a transmission/reception direction with respect to a mobile station with a single antenna. The antenna array can be considered in two parts: a transmission antenna array and a reception antenna array. The transmission antenna array is focused on for description by way of example. Yet, the hardware of the antenna array is commonly used for transmission and reception.
A TDD (Time Division Duplex) system, since it uses an identical frequency band for transmission and reception, shows the same characteristics in transmission and reception and applies a weight vector obtained for a reception antenna array operation to a transmission antenna array operation as well. On the other hand, an FDD (Frequency Division Duplex) system calculates a weight vector separately for a transmission antenna array because a transmission frequency band is separated from a reception frequency band by a coherence bandwidth or greater. It is to be appreciated that the following description is made on a transmission antenna array system of an FDD system.
Blind transmission is characteristic of transmission antenna arrays that have been developed so far. The blind transmission refers to transmission without receiving any feedback information of the channel characteristics of a forward link from a mobile station. These transmission antennas operate based on the following reciprocity suppositions between transmission and reception channels.
Supposition 1: a forward fading channel and a reverse fading channel arrive at their destinations from the same number of paths and transmission and reception occur in the same path direction.
Supposition 2: if the difference between a transmission frequency band and a reception frequency band is greater than a coherence bandwidth in an FDD system, the forward and reverse channels have mutually independent instant fading coefficients but an identical average fading power for the same path.
Raleigh has suggested a blind transmit antenna array for a single fading path as shown in FIG. 2 (reference 1: G. G. Raleigh and V. K. Johnes, “Adaptive Antenna Transmission for Frequency Duplex Digital Wireless Communication,” in Proc. IEEE ICC, pp. 641–646, Montreal, Canada, June 1997).
A channel vector refers to a collection of the vector-expressed characteristics of each antenna in a transmit antenna array with respect to a reception antenna. If we let a forward channel vector be h, then h=βa(θ). β is a fading coefficient independent of a reverse fading coefficient according to supposition 2, θ is a transmission direction from a base station to a mobile station, which the base station knows from a reverse signal by supposition 1 without receiving forward fading feedback information from the mobile station, and a(θ)—corrected is an array vector for the direction θ.
The base station transmits a transmission message s(t) by forming a beam with a weight vector w and the message s(t) arrives at the mobile station on a forward channel h. The received signal r(t) can be expressed byr(t)=hTws(t)+n(t)  (2)where n(t) is additive white Gaussian noise (AWGN).
According to a matching filter theory, an optimal weight vector that brings a maximal output SNR at a receiving end of the mobile station is
                              w          _                =                              P                    ⁢                                    h              *                                                                    h                _                                                                                      (        3        )            where P is the transmission power of the base station, * is a conjugate operator, and ∥·∥ is the norm of a corresponding vector. By applying the relationship of h=βaθ to Eq. 3,
                              w          _                =                              P                    ⁢                                                                                          a                    _                                    *                                ⁡                                  (                  θ                  )                                                                                                                  a                    _                                    ⁡                                      (                    θ                    )                                                                                        .                                              (        4        )            
From Eq. 4, it is noted that an optimal weight vector is set using only the transmission direction θ known from a reverse signal by supposition 1 without a fading coefficient. Because a single path is assumed, not a fading coefficient but an array vector is obtained.
Now, a description of the transmission antenna array suggested by Raleigh will be given. Referring to FIG. 2, a transmission message is propagated in the air via an antenna array 203 by a beam formed in a specific transmission direction in a transmission beam generator 202. A reverse processor 205 processes a reverse channel signal received via the antenna array 203. An array vector calculator 207 divides reversely received signals for each path through a path divider in a rake receiver of the reverse processor 205 and calculates a direction (array vector) on the basis of direction information of the received signals. A weight vector calculator 209 calculates a weight vector using the array vector and outputs the array vector to the transmission beam generator 202. The transmission beam generator 202 controls generation of a transmission beam by assigning a weight to a transmission signal that is to be output via a corresponding antenna based on the weight vector.
The above transmission antenna array system estimates the reception direction of a signal received via the antenna array 203, calculates a weight vector (array vector) for the transmission antenna array based on the estimated direction information, and then forms a transmission beam using the weight vector, for transmission.
Despite the advantage of simple structure, the Raleigh transmission antenna array using a single path is not feasible for a multi-path system.
Thompson has suggested a blind transmission antenna array with a multi-fading path as shown in FIG. 3 (reference 2: J. S. Thompson, J. E. Hudson, P. M. Grant, and B. Mulgrew, “CDMA Downlink Beamforming for Frequency Selective Channels,” PIMRC'99, B2-3, Osaka, Japan, September 1999).
In the case of a multi-fading path (M paths), a reception direction for each path must be estimated from an input signal to form a forward transmission beam as is done in the case of a single path. If a reception direction (a transmission direction according to supposition 1) for an ith fading path (i=1, 2, . . . , M) is θi, a transmission beam for the ith fading path is formed in the direction of θi. The issue is how to determine weights (different from weight vectors). Considering this issue, an optimal weight vector is determined in the following way.
Assuming that the base station transmits a transmission message by forming a transmission beam with a weight vector w and it arrives at the mobile station from three different paths on a forward channel, a signal r(t) received at the mobile station can be expressed byr(t)=h1Tws(t−τ1)+h2Tws (t−τ3)  (5)where τi is a propagation delay for an ith path and h1 is a channel vector for an ith path. Similarly to a single path, with respect to the transmission direction θi and a fading coefficient βi, h1 is as follows. Herein, the fading coefficient βI means a value including a phase and a size value of the received signal.h1=β1a(θ1)  (6).
According to the matching filter theory, an optimal weight vector that brings a maximum output SNR at a receiving end of the mobile station iswo=argwmaxwHHHHwsubject to ∥w∥2=P  (7)
where H=[h1h2h3]
where P is transmission power, wOis an optimal weight vector, and h1,h2,h3 are channel vectors for the paths. The solution of Eq. 7 is set as a maximum unique vector corresponding to the maximum unique value of a transmission correlation matrix
            H      H        ⁢    H    =            ∑              i        =        1            3        ⁢                                                  β            i                                    2            ⁢                        a          _                ⁡                  (                      θ            i                    )                    ⁢                          ⁢                                                  a              _                        ⁡                          (                              θ                i                            )                                H                .            
From the foregoing, it can be noted that the base station needs to know a fading coefficient {βi} as well as a transmission direction {θi} in order to achieve the optimal weight vector. On the contrary, the base station need not know a fading coefficient to form a transmission beam for a single fading path. In an FDD environment, the instant fading coefficient of a reverse channel is different from that of a forward channel. Thus, it is no use analyzing a received reverse signal to obtain the instant fading coefficient of the forward channel.
By replacing HHH of Eq. 7 with an expectation E[HHH], Thompson proposed a semi-optimal weight vector given by
                              E          ⁢                                          [                                    H              H                        ⁢            H                    ]                =                              ∑                          i              =              1                        3                    ⁢                                    E              ⁢                                                          [                                                                                      β                    i                                                                    2                            ]                        ⁢                                          a                _                            ⁡                              (                                  θ                  i                                )                                      ⁢                                                                                a                    _                                    ⁡                                      (                                          θ                      i                                        )                                                  H                            .                                                          (        8        )            
In Eq. 8, the transmission direction {θi} (the array vector {a(θ)i}) is estimated from a received reverse signal according to supposition 1 and E[|βi|2] is also estimated from the received reverse signal according to supposition 2.
This is blind beam formation without the need of receiving feedback information about a fading coefficient from a mobile station. However, the blind beam formation has a slightly lower performance than the non-blind beam formation using an optimal weight vector calculated by Eq. 7.
FIG. 3 is a block diagram of the transmit antenna array device suggested by Thompson. Referring to FIG. 3, a transmission message is formed into a beam by a transmission beam generator 302 of a forward processor 301 and propagated in the air in a particular direction via an antenna array 303. A reverse processor 305 processes a reverse channel signal received via the antenna array 303. A forward fading power calculator 307 estimates a fading coefficient of the received signal for each path, which is obtained by the reverse processor 305 in the course of processing the received signals and calculates the average power of the estimated fading coefficients. The reverse average fading power is calculated based on supposition 2. An array vector calculator 309 divides the received signals for each path through a path divider in a rake receiver of the reverse processor 305 and calculates the input direction (array vector) of the received signal from the received signals. A transmission correlation matrix calculator 311 obtains a transmission correlation matrix using the average fading power and the array vector and a weight vector calculator 313 calculates a weight vector using the transmission correlation matrix. The transmission beam generator 302 assigns a weight to a transmission signal that will be output via a corresponding antenna according to the weight vector received from the weight vector calculator 313, to thereby control formation of the transmission beam.
According to the Thompson transmit antenna array system, a reception antenna array first estimates the input direction (array vector) of a received signal. Then, the reception antenna array estimates a fading coefficient of the received signal for each path and calculates the average power of the fading coefficients. Based on the direction information and the average fading power information, a weight vector for a transmission antenna array is calculated. Finally, a transmission beam is formed using the weight vector and transmitted.
While the Thompson antenna array structure can be used as a transmission antenna array system in a multi-path environment, the use of an average fading power makes it impossible to calculate a precise weight vector. That is, the average fading power is used in calculating a forward fading power instead of an instant fading power. The average fading power is calculated based on supposition 2. A reverse average fading power is calculated from a received signal for use as an average forward fading power. The limitation of the Thompson antenna array in calculating a precise weight vector decreases the performance of the antenna array system